The phase transition behaviour of linear polyethylenes at high pressure

The influence of the pressure on the melting behaviour of different linear polyethylene samples with molar masses from 5 to 4000 kg mol(-1) has been investigated by three methods: crystallization and melting at different pressure in the high pressure DSC, crystallization at different pressure and subsequent melting at normal pressure, crystallization at high pressure (500 MPa) and melting at normal pressure but at different heating rates. The dependence of melting temperature and enthalpy change on crystallization pressure are given. The high pressure crystallized material seems to be in a metastable state (after decompression) with larger enthalpy and higher melting temperature than in the equilibrium state. The high molar mass sample behaves like a single component system, whereas that with low molar mass shows a complex multicomponent phase separation behaviour. (C) 1997 Elsevier Science B.V.